But if physicians are correct, their fates already have been determined. The term brain death, by definition, means the patient will not recover, said David Greer, chair of neurology at Boston University School of Medicine.

"If the diagnosis has been made correctly, there's never been any case that has been legitimized or validated that there's been any recovery of brain function," said Greer, also the chief of neurology at Boston Medical Center.

The boys' families have gone to court to stop attempts by CHOP to remove the pair from life support. So far their lawyer, Christopher F. Bagnato, has bought them time to look for facilities that will take in the boys. On Wednesday, an appellate panel delayed any decision in Jayden's case until at least May 30. Areen's case, meanwhile, is pending in the Orphans Court division of Common Pleas Court.

"We're still trying to get these boys out," Bagnato said. "The families do not think the boys are dead. They have a different definition of death."

CHOP has declined to comment on the two cases, citing laws that restrict the release of patient information.

But, in general, an evaluation to determine brain death involves conducting a series of evidence-based tests, said Greer, who co-authored a 2010 update of the guidelines on adult brain death for the American Academy of Neurology.

The pediatric guidelines are slightly different, but the tests typically include checking for a response after touching the patient's eye with a piece of tissue paper, shining a light on the pupils, and irrigating the ears with ice water. If there is any doubt, specialized equipment can then be used to measure activity and blood flow in the brain, according to pediatric guidelines that were updated in 2011.

In the hands of a trained neurologist, a finding of brain death based on these exams is rock-solid, Greer said.

In making that statement, he included the controversial case of Jahi McMath, an Oakland, Calif., teenager who was the subject of a lengthy article February in the New Yorker. She was diagnosed as brain dead, but family members maintain she has recovered slightly, making small movements in response to their spoken words. That view has the support of D. Alan Shewmon, a retired UCLA neurologist who examined the girl and has watched videos of her made by the family where they now live, in an apartment outside New Brunswick, N.J., the magazine said.

But videos can be misleading, and in the absence of formal testing and a peer-reviewed publication of any findings, the neurology community at large is not convinced, Greer said. A patient can make slight movements due to spinal reflexes even when there is no brain function, he said.

"The movements that are being described [in McMath], it's not really clear that it's coming from the patient" as the result of brain function, Greer said. "This really needs to be very carefully scrutinized."

Rumpa Banerjee, the mother of 14-year-old Areen, has read about the McMath case, as well, and it gives her hope. She acknowledged that Areen's outlook seemed bleak at first, but said she has seen signs that he responds to her presence. When she speaks to her son in his hospital room, his blood pressure has lowered in response, she said.

"He is a very strong boy," she said Thursday. "It's my belief if he is given the brain-dead exam again, it might show that he is recovering."

Whatever such testing might show, legal questions remain unsettled.

As most states do, Pennsylvania allows a physician to declare a patient legally dead if either heart or brain function has completely and irreversibly stopped, said Thaddeus Pope, a professor at Mitchell Hamline School of Law in St. Paul, Minn.

But in New Jersey, lawmakers have enacted a religious exemption. If a patient's faith dictates that life persists so long as the heart is beating, then brain death alone is not sufficient for a legal declaration of death, Pope said.

Bagnato said such an exemption would apply in his two cases, as Areen's family is Hindu, while Jayden's mother is Buddhist and his father is Catholic.

For now, they remain in Pennsylvania. But Bagnato said both families are making progress in finding New Jersey facilities that might take the boys, growing close to each other as they share the details of their medical and legal ordeals.